In a control system comprising a single control unit and a plurality of controlled apparatuses (including: a controlled part for operating in accordance with an instruction from the control unit; and a sensor part for sending information to the control unit), a technique for decreasing the number of wires, or so-called simplification, has been widely performed. As a typical technique for the simplification, a method has been widely employed, in which a master station and a plurality of slave stations that have a conversion function of parallel and serial signals are connected to a control unit and a plurality of controlled apparatuses, respectively, whereby data is sent and received between the master station and the slave stations through a serial signal, instead of parallel connection by which individual signal lines extending from the controlled apparatuses are directly connected to the control unit.
In realizing the simplification with many slave stations connected, if a location of a disconnection cannot be identified from the side of a control unit, it is necessary to check the wiring of each slave station positioned far away from the control unit. In this case, there needs to be a lot of steps to identify a location of a disconnection.
Accordingly, there is proposed a method of confirming a disconnection on the side of slave stations from the side of a control unit. For example, Japanese Unexamined Patent Application Publication No. S62-173830 discloses a disconnection detection method for a time-division multiplex transmission system, in which: a plurality of terminal parts with a preset address are connected to a single master part via a common signal line; and the master part sequentially transmits transmission signals while cyclically changing the designated addresses and receives return signals from the designated terminal parts. In this disconnection detection method, when the master part detects that a state of being unable to receive the return signal from any terminal part is continuously generated over a predetermined number of transmission cycles, the occurrence of a disconnection of single line is judged.
This disconnection detection method employs, as a scheme for transmitting or receiving data between a plurality of apparatuses, a command scheme in which a partner for sending or receiving data (slave station) is designated by data for identifying the partner (address data). This command scheme can be employed even in a simplification system in which data is sent and received through a serial signal. However, the command scheme has a problem that the transmission efficiency is reduced when the amount of command data (address data) exceeds that of data for a transmission purpose. In order to increase the transmission efficiency of a serial signal, a transmission synchronization scheme may be preferred in which data is sent and received through a monitor signal and a control signal sequentially sent in a pulse cycle that does not contain address data. In the simplification techniques, the command scheme and the transmission synchronization scheme are selectively used depending on an application. Even for systems employing the transmission synchronization scheme, there is proposed a method of confirming a disconnection on the side of slave stations, from the side of a control unit.
For example, Japanese Unexamined Patent Application Publication No. H07-099503 discloses a data transmission apparatus that can easily identify a location of an abnormality in a transmission path. In this data transmission apparatus, each terminal part is provided with answer means. This answer means outputs a confirmation signal to an end region of pulse signals in one cycle to be transmitted to a single line when a count value in a cycle counter for the slave station becomes a preset value assigned thereto. Then, abnormality detection means connected to the signal line determines the presence or absence of the confirmation signal that corresponds to the count value in the cycle counter thereof and that is transmitted from the answer means of any terminal part. If the confirmation signal is present, the abnormality detection means determines that there is no connection abnormality in the terminal part corresponding to the counter value, and outputs a check signal to the end region of the pulse signals in one cycle to be transmitted to the signal line. If the confirmation signal is absent, the abnormality detection means determines that there is a connection abnormality in the terminal part corresponding to the counter value, and does not output the check signal. This enables a master part to detect a disconnection.
Moreover, Japanese Unexamined Patent Application Publication No. H09-084155 discloses, in a control/monitor system which includes a single primary station and one or more secondary stations branch-connected to the primary station; controls the secondary stations by transmitting control signals sent out from the primary station to the secondary stations; and transmits a monitor signal containing a state of each secondary station to the primary station, a technique that enables the central primary station to detect a location of an abnormality, and to detect a disconnection even when a transmission cable is branched, thereby identifying states of apparatuses, such as a controlled part, a sensor part, and the like, at the center. In the system disclosed in this document, each frame sent out from the primary station includes a start signal with a long period of a power source voltage level, a data signal, a signal of an identification code (ID code) of each secondary station, and an end signal, and these frames are sequentially sent out while the ID codes are changed by turns. In each secondary station, an ID cord assigned to the secondary station and a counter value indicating a sending/receiving address of data are preset. If this ID cord matches that received from the primary station, each secondary station generates an end response at a timing of the end signal. In response, the primary station stores the end response from a corresponding one of the secondary stations, and records the configuration of the secondary stations.